Hybrid AD beamforming conceptually relies on the decomposition of the
beamforming operation as the product of a low-dimensional digital
beamforming matrix and a high-dimensional analog beamforming matrix.
The elements of the analog beamforming matrix obey the unit-modulus
constraint imposed by the phase shifters. To reduce the number of RF
chains, several authors [1]}, [2]} advocated
antenna-selection based hybrid beamforming. In order to further
improve their performance, spatially sparse precoding techniques were
developed by exploiting the millimeter-wave (mmWave) channel
characteristics in [3]}, [4]}. Moreover,
the authors of [5]} demonstrated that hybrid AD beamforming
having twice as many RF chains as data streams approaches the
performance of FD beamforming.
